The Histiocytoses are rare non-malignant disorders of the mononuclear phagocytic system. The pathophysiology of many of these diseases is unknown and, in the case of the widely- disseminated and potentially fatal forms, treatments to date have been variably effective and sometimes highly toxic. Histiocytoses meet criteria for designation as orphan diseases. The Histiocyte Society, founded 26 years ago, is the unique international organization of clinicians and scientists focused on expanding knowledge about these diseases and improving treatment outcomes. A major focus of activities is the annual meeting which changes venues around the world. The scientific agenda includes presentations by invited speakers regarding the most recent developments in the biology of histiocytic disorders, as well as topics in more basic hematology and immunology related to the development of macrophages and dendritic cells, and mechanisms of immune homeostasis. The program also has several platform sessions and a moderated poster presentation of selected abstracts which have been scored by the Educational Committee of the society. Additionally, the meeting allows for face to face meetings of the Executive Board of the society, the Educational and Scientific committees, LCH and HLH Disease Interest Groups, and clinical study groups that manage the society-sponsored clinical trials, and the Data Safety Monitoring Board. PUBLIC HEALTH RELEVANCE: The Histiocytic Disorders, while individually rare, as a group represent a significant burden to the health care system due to the complexity and cost of managing children with such disorders. Adult patients with histiocytosis are particularly underserved. Treatments which are effective in children are often too toxic for older patients. The Histiocyte Society annual meetings are the only opportunity to gather medical practitioners from around the world working together to advance knowledge which leads to improved treatment outcomes in these patients.